1. Field of the Invention
The present invention relates generally to wavelength routed communications networks, and more specifically to an optical cross-connect system for switching wavelength division multiplexed signals.
2. Description of the Related Art
A known optical cross-connect system, as described in a paper "Reconfigurable Wavelength Routed Optical Networks: a Field Demonstration", H. J. Westlake at al., ECOC '91 proceedings Vol. 1, pages 753 to 756, includes an optical space switch, a plurality of wavelength division demultiplexers, and a plurality of wavelength division multiplexers. As shown in FIG. 1, wavelength time division multiplexed (WTDM) channels on incoming fiber optic trunks are demultiplexed into wavelength-divided channels by demultiplexers 2 and appear at the inlet ports of the optical space switch 1. Some of the wavelength-divided channels are switched to the wavelength division multiplexers 3 and then routed to outgoing fiber optic trunks, and others are routed to an electronic cross-connect system 4 where they are time-division demultiplexed for add/drop multiplexing. Links are established in the switch 1 so that the wavelength numbers on the inlet side correspond to those of the outlet side. If a fault occurs on an outgoing trunk 5 and a spare outgoing trunk 6 is available, links x1, y1 and z1 leading to the trunk 5 will be cleared and new links x2, y2 and z2 established to the spare trunk 6. Since this fault recovery procedure involves one or more downstream nodes and a close coordination is necessary to determine the alternate links in the associated nodes, and a complex, inefficient recovery procedure will result. If it is desired to restore all of the failed links, it will be necessary to provide as many spare wavelength-divided channels as there are working wavelength-divided channels.